The present invention relates to a light-sensitive recording material composed of an electrically conductive support, suitable for the production of a printing form or printed circuit, and a photo-conductive layer system containing a photoconductor, a binder, a sensitizing dye and conventional additives. The present invention further relates to the use of the aforesaid recording material in a process for the production of a printing form or printed circuit.
Processes have been disclosed which exploit the high light sensitivity of electrophotographic systems for the production of images. A particular advantage of such processes is that the image can be produced by laser exposure or projection exposure. A film original is not necessary.
The production of planographic printing plates by electrophotographic means has been disclosed by German Pat. No. 1,117,391, corresponding to British Pat. No. 944,126. The disclosed printing plates are provided with a coating which, in addition to a suitable binder and an appropriate sensitizing dye, contains an organic photoconductor. A latent charge image, which is rendered visible by means of a dry or liquid developer, is produced on this layer by electrostatic charging and imagewise exposure. The toner image obtained is fixed, for example by heating the plate, so that, in a final decoating step, the toner image areas are protected from attack by the decoating solution.
However, such a process still has disadvantages. For example, the fixed toner must either be washed off the printing plate surface by means of a solvent, or an increased number of waste sheets at the start of printing must be accepted, when the toner only "runs off" in the printing machine. The achievable resolution is also limited, since baking of the toner leads to undesirable shifts in tonal value. Moreover, the temperature in the baking phase must be kept within the optimum range, with considerable expense entailed in the controls. Finally, relatively long runs are not obtained, since the printing image areas do not withstand mechanical stress for a prolonged period.
Attempts have already been made to combine the advantages of the electrophotographic process described with those of conventional positive- or negative-working systems. German Pat. No. 1,447,008, corresponding to British Pat. No. 996,315, has disclosed the production of a planographic printing plate which was coated with a mixture of organic photoconductors and the constituents of conventional positive-working layers based on o-quinonediazide. The toner image produced electrophotographically on this layer was used as a photomask in a further exposure step with actinic light. The differentiation of solubility between the image areas and non-image areas was induced via the photochemical reaction of the exposed o-quinonediazide. The disadvantage in this process, however, is that the conductivity of the above-mentioned coating in the dark is high, and that the solubility differences between the image areas and non-image areas are limited because of the relatively high photoconductor content.
A negative-working photoconductor layer has been disclosed by German Pat. No. 1,811,209, corresponding to British Pat. No. 1,274,296. In this case, a recording material is used which, in the photoconductive layer, contains a photopolymerizable photoconductor, selected from the vinyloxazoles and vinylcarbazoles. The disadvantage in this approach is that, even with the high photoconductor content of the layer, the hardening of the areas not covered by toner still does not meet more stringent requirements, in spite of a relatively high light intensity.
In order to avoid the disadvantages of these processes, attempts have also been made to apply a photoconductor layer to the light-sensitive layer. Such double layers are described in German Pat. No. 1,071,478, corresponding to U.S. Pat. No. 2,939,787, in European Patent Application No. 0,053,362 and in German Offenlegungsschrift No. 3,310,804, corresponding to British Pat. No. 2,121,201. In all cases, a photoconductor layer is applied as a covering layer to a conventionally positive-working or negative-working light-sensitive coating. Such a procedure is similar to an electrophotographic process, because the toner image can then be produced on the surface of the photoconductor layer.
But this combination of layers has a number of disadvantages. On the one hand, there is an increased risk of halations, since the photoconductor mask formed by the toner image and the light-sensitive base layer are separated from one another by the photoconductor layer. The photoconductor layer must therefore be as thin as possible. On the other hand, the layer may contain only those photoconductors and sensitizer dyes which are transparent to radiation of those wavelengths which are suitable for a photoreaction in the light-sensitive layer. Moreover, when the photoconductor layer and a negative-working layer are combined, the photoconductor layer is also removed from the image areas on decoating. It is therefore no longer available for the further processing steps. This is also true for the combination of photoconductor layer and a positive-working layer, if the applied toner image is not fixed. If the toner image is fixed, however, there results an undesired toner run-off at the start of printing, as mentioned above. A final disadvantage is that, in cases where the interface between the photoconductor layer and the lightsensitive layer is not completely uniform, a layer of varying thickness is obtained after decoating, and this can lead to premature wear of the printing form produced thereby, or to a non-uniform reproduction of halftone originals.